The indexer verifier should create an error when an evar is used only once (a singleton variable).
This is almost always a typo.
One exception: when a variable is followed by "?".
(You might want to have a flag to control whether this is an error (non-zero return code) or a warning (zero return code).

Maybe we could extend the Var? syntax to include Var! meaning "I intend this to be a singleton and it's OK", and otherwise it's an error? That seems like it would avoid the common case where it's accidental, but gives the test author an easy escape.

There's a general convention in Prolog that names starting with "_" (including "_" by itself) are treated as "it's OK to be a singleton". I don't think that this is used often, but I do recall using it, mainly as a documentation thing. (especially as the verifier doesn't have any comment syntax of its own, this could be useful)